1. Technical Field
This invention relates generally to a method of forming phase change memory devices.
2. Description of the Related Art
Phase change memory devices use phase change materials, i.e., materials that may be electrically switched between a generally amorphous and a generally crystalline state or between different detectable states of local order across the entire spectrum between completely amorphous and completely crystalline states, for electronic memory application. The state of the phase change materials is also non-volatile in that, when set in either a crystalline, semi-crystalline, amorphous, or semi-amorphous state representing a resistance value, that value is retained until changed by another programming event, as that value represents a phase or physical state of the material (e.g., crystalline or amorphous). The state is unaffected by removing electrical power.
At present, alloys of group VI of the periodic table, such as Te or Se, referred to as chalcogenides or chalcogenic materials, can advantageously be used in phase change cells as phase change materials. One chalcogenide is formed by a Ge, Sb and Te alloy (Ge2Sb2Te5), also called GST, which is currently widely used for storing information in overwritable disks.
One step in the process for manufacturing memory devices is the deposition of the GST or chalcogenic layer. In fact, a common deposition technique, like conventional sputtering, is difficult to use for depositing layers of calcogenides, since the sputtering process could stop after a while or arcs could occur.
Another problem resides in the fact that for a proper functioning of the memory device, it may be necessary that the GST layer be deposited conformally, in particular, that a sufficiently thick, uniform GST layer is conformally deposited in openings, trenches or pores; furthermore, it is important that no void areas are formed.
However, the above conditions are difficult to obtain in case of a GST layer having a high thickness (e.g., greater than 120 nm) and/or when the openings, trenches or pores have a high aspect ratio (that is high depth compared with the width thereof).
Another step that may be improved in the process for manufacturing memory devices is the deposition of electrodes of carbon films by CVD (chemical vapor deposition). In fact, in this case the carbon target is hard to ignite.